1. Field of the Invention
The invention is directed to improving color and texture of green vegetables that are containerized and sterilized. A high-concentration short-time zinc blanch (HCST) method is introduced through which the bright green color of fresh vegetables is maintained through the thermal sterilization process.
2. The Prior Art
Vegetables and vegetable products, such as soups, are containerized and thermally sterilized in order to attain shelf life stability. This obvious advantage of shelf life stability brings along inherent disadvantages for the product, such as changes in flavor, texture, and color.
Visually most noticeable among the temperature-induced thermal processing changes may be the color change of green vegetables, from bright green to an unappealing olive color. At the biochemical basis of this color change is the chlorophyll molecule, the molecule responsible for the green color of vegetables. Its structure is that of a complex molecule with a magnesium atom in the central position. Accumulative heat input, either through extended cooking or through thermal sterilization in containers, causes the magnesium atom to be displaced from its central position. This change at the molecular level, from chlorophyll to pheophytin, coincides with a color change in the vegetable, from bright green to olive (Gold & Weckel, 1959; MacKinney & West, 1940; Westcott et al., 1955).
While the magnesium loss from the chlorophyll molecule is irreversible, the change in color is not. Spontaneous "regreening" of green vegetables was first reported in canned okra in 1943 (Fischbach and Newburger) and was attributed to zinc-containing pigments (Fischbach, 1943). Zinc and copper complexes of pheophytins formed and regreened pea puree during storage (Schanderl et al., 1965). Copper or zinc ions, as contaminants in process solutions, caused sporadic bright green color in processed green beans (Decleire, 1966) and Brussel sprouts (Swirski et al., 1969). Regreening through formation of zinc or copper complexes has also been shown experimentally (Jones et al., 1977; vonElbe et al., 1986).
The addition of zinc or copper salts for the retention of green color in canned vegetables has been reported in several prior art processes, all of which include one or more serious obstacles to implementation in a modern vegetable processing operation.
For example, U.S. Pat. No. 5,114,725 describes a lengthy process in which vegetables are first blanched and then containerized with a brine solution containing zinc and/or copper ions. Subsequently, the canned vegetables are held at an elevated temperature for 15 to 90 minutes after which they are sterilized according to commercial guidelines. This sterilization process may be followed by additional thermal treatments to further enhance the green color. It is suggested that vegetables preserved in such a manner may be used as ingredients in secondary products. This process has several drawbacks, the most important ones being the lengthy processing time and the impracticality of having to open cans to obtain ingredients for secondary products such as soups. Neither visual color ratings nor instrumental results are provided to support the claim of regreened vegetable color. In addition, no mention is made of a comparison of the regreened color to that of the fresh vegetable.
Similarly, U.S. Pat. No. 5,482,727 is characterized by a lengthy and complex process. Two different methods are presented. In the first one, vegetables are subjected to an extended pre-blanch hold of up to 60 minutes duration, then packed into cans with zinc or copper solution, and sterilized. The second method consists of an elevated temperature hold of the vegetables in water, followed by a blanch in zinc or copper ion solution, and subsequent packing and sterilizing of the vegetables in cans. In order to increase efficiency of the zinc blanch, acidification to pH 5 and the use of deionized water are advised, yet two more hurdles to the simplification that is desirable in a commercial processing operation.
In contrast, U.S. Pat. No. 4,473,591 provides a simplified method having reduced processing time for the preservation of green color in vegetables. Examples are directed to treatments of green beans, green peas, and spinach, which are separately blanched in a zinc or copper ion solution for 5 to 6 minutes, after which they are packed into cans and sterilized. Blanch water metal ion concentrations of 50 to 500 ppm are employed in this method. Although the described method appears to be much more practical than any of the others above, even 5 minute blanch times exceed those practiced in today's commercial vegetable blanching operations.
A color preservation blanch method that is to be used economically at a modern vegetable producer must accommodate large production volumes, without introducing unnecessary delay. Preferably, the method is accommodated within the extremely short blanch times in continuous blanching equipment. For instance, broccoli blanch times at a commercial facility typically range from 65 to 105 seconds, while green bean and green pepper blanch times vary between 1.5 and 2.5 minutes.
The second most noticeable effect of temperature is that it changes texture. This change typically softening, is, to a certain extent, desirable, since cooked vegetables are easier consumed and digested than raw vegetables. However, the degree of softening that is achieved through commercial sterilization processes exceeds most consumer's sensory preference.
Fresh vegetables owe their crispness largely to complex pectin molecules in the cell walls. Stanley et al. (1995) describe that the heating of vegetables causes cell separation due to thermal destablilization of pectic materials. The complex pectins are broken into subunits, a process that is concurrent with structure loss. As for the above described color change, this texture change is preventable. The endogenous pectin methyl esterase (PME) has the potential of cross-linking the pectin subunits with calcium ions, which are present in the vegetable tissue and can also be added via the blanch solution. Thus, calcium ion "bridges" between the subunits form complex pectin polymers, yielding a product of increased firmness.
Two prior art processes for texture improvement of vegetables to be frozen use the above described firmness enhancing potential of pectin methyl esterase. U.S. Pat. No. 4,521,439 describes a process of three consecutive blanch steps: First, a 1 to 30 second steam blanch at 90 to 100.degree. C.; second, a 1 to 60 minute water blanch at 45 to 90.degree. C.; and optionally a third step consisting of a 30 second to 5 minute blanch between 80 to 100.degree. C. Vegetables treated in such manner were frozen and later compared to conventionally blanched and frozen control vegetables. After cooking vegetables from both treatments, the vegetables blanched in the three-step process had improved texture over the texture of the conventionally treated vegetables.
U.S. Pat. No. 5,607,712 describes a two step procedure for the texture improvement of vegetables that are frozen and later canned. Here the steps consist of (1), a 2 to 60 minute low temperature blanch at 125 to 155.degree. F. for pectin methyl esterase activation, and (2), a 2 to 10 minute blanch at 190 to 210.degree. F. for enzyme deactivation. The observed firming effect is said to be accentuated through the addition of calcium salt and/or acid to the blanch water of the first step. This process is limited to the use of fresh vegetables that are to be frozen and later canned.
None of the prior art processes for the purpose of green color retention describe any effects on texture. It is believed that the low zinc ion concentrations used in the prior art processes do not provide texture-enhancing effects.